Abstract

A wide variety of trace gases (e.g. dimethyl sulphide, organohalogens, ammonia, non-methane and oxygenated hydrocarbons, volatile oxygenated organics and nitrous oxide) are formed in marine waters by biological and photochemical processes. This leads in many, but not all, cases to supersaturation of the water relative to marine air concentrations and a net flux of trace gas to the atmosphere. Since the gases are often in their reduced forms in the water, once in the atmosphere they are subject to oxidation by photolysis or radical attack to form chemically reactive species that can affect the oxidizing capacity of the air. They can also lead to the formation of new particles or the growth of existing ones that can then contribute to both direct and indirect (via the formation of cloud condensation nuclei) aerosol effects on climate. These cycles are discussed with respect to their impacts on the chemistry of the atmosphere, climate and human health.

This whole topic was the subject of an extensive review (Nightingale & Liss 2003 In Treatise in geochemistry (eds H. D. Holland & K. K. Turekian), pp. 49–81) and what will be attempted here is a brief update of the earlier paper. There is no attempt to be comprehensive either in terms of gases covered or to give a complete review of all the recent literature. It is a personal view of recent advances both from my own research group as well as significant work from others. Questions raised at the meeting ‘Trace gas biogeochemistry and global change’ are dealt with at appropriate places in the text (rather than at the end of the piece). Discussion of each of the gases or group of gases is given in the following separate sections.

Keywords:

1. Dimethyl sulphide

Dimethyl sulphide (DMS) is a gas that is biogenically produced by algae in marine waters. It is formed by cleavage of the precursor molecule dimethylsulphoniopropionate (DMSP) that acts as an osmolyte and cryoprotectant for the organisms. A small fraction (probably less than 10%) of the total DMS formed in seawater is transferred to the atmosphere across the air–sea interface. In the atmosphere, it is unstable and converted into a variety of products some of which are particulate, making DMS a major natural provider of fine particles over the open oceans. In this way, it plays an important role in climate as a source of cloud condensation nuclei (CCN), as well as supplying the element sulphur to the terrestrial environment, where it is essential for plant growth (e.g. Zhao et al. 1999).

There is a substantial literature about the complex cycle within the surface oceans of the production and destruction processes of DMS and DMSP (e.g. Stefels et al. (in press) for an up to date review, particularly aimed at how best to incorporate the various processes in models). A significant recent paper on the role and importance of DMS for organisms is that by Sunda et al. (2002). These authors show from laboratory experiments that DMS in water can act as a scavenger of hydroxyl and other reactive oxygen species. They found that factors which induce oxidative stress, including solar UV radiation, CO2 or Fe limitation, Cu2+ and H2O2, all led to enhanced DMSP/DMS production in cultures of marine algae. This finding has significantly increased the range of factors that have to be included in any quantitative assessment (and modelling) of DMS emissions to the atmosphere.

Another factor leading to DMS release from algal cells is viral infection (e.g. Malin et al. 1998). In a recent paper, Evans et al. (2006) show that addition of DMS and/or acrylic acid (the equimolar products of DMSP cleavage) at realistic concentrations inhibit the viral infection of Emiliania huxleyi, whereas addition of DMSP or DMSO (and oxidation product of DMS) has no effect. It was also shown that the DMS and acrylic acid showed greater antiviral effect when added together rather than separately. The implication of these results is that the DMSP system may act as a chemical defence against viral attack thus protecting the remaining cells from further infection.

Recent papers, including those mentioned previously, illustrate the considerable complexity of the processes by which DMSP and DMS are produced and transformed within oceanic surface waters. In order to unravel some of this complexity, attempts are being made to use molecular techniques to identify the genes controlling particular processes. A recent example is the paper by Howard et al. (2006) in which a bacterial gene, dmdA, isolated from a Roseobacter strain (Silicibacter pomeroyi), was shown to catalyse the first vital step in the demethylation of DMSP. Further, by searching in gene libraries, the authors found that dmdA-like genes are present in other bacterial taxonomic groups and speculate that one-third of all marine bacterioplankton may be able to demethylate DMSP.

In addition to the functions of osmoregulation, cryoprotection and as an antiviral and antioxidant agent, it has been proposed that DMS can also act as a signalling compound between organisms. Nevitt et al. (1995) showed that the foraging behaviour of sea birds was affected by the presence of artificially added DMS. They reasoned that since DMS is known to be released when DMSP-containing phytoplankton are grazed by zooplankton, the ability to sense DMS would give the birds an advantage when searching for zooplankton-rich food sources. At a smaller scale, Steinke et al. (2006) have recently reported on laboratory experiments in which they use video microscopy to show that a tethered copepod (Temora longicornis) was able to detect the presence of micromolar concentrations of DMS injected into the water as adjudged by the frequency of its ‘tail flapping’.

The productivity of major areas of the oceans has been shown to limited by the availability of iron, mainly via experiments at sea in which patches of water have been enriched (fertilized) with iron sulphate (Boyd et al. 2007). The response to Fe addition in terms of increases in DMS (and DMSP) is that threefold increases in DMS are typical although up to eightfold elevations have been observed (Turner et al. 2004). However, this is not always the case, since Levasseur et al. (2006) report lower DMS but elevated DMSP in an iron fertilization study in the northeast Pacific. In general, iron enrichment tends to particularly benefit diatoms, which are considered to be poor DMSP producers, so that the size of any DMS increase is often less than that for a general productivity indicator such as chlorophyll.

In the atmosphere, DMS is subject to oxidation by free radicals including OH and NO3 to yield a number of gaseous and particulate products. A recent modelling paper by von Glasow et al. (2004) argues for the importance of bromine oxide (BrO) as an additional oxidant. Their model results indicate that typical levels of BrO in the marine atmosphere may lead to significant oxidation of DMS which could reduce tropospheric column concentrations by 60% and reduce particle production/growth from DMS and thus its potential to affect climate.

Coupling of ocean and atmosphere models has been attempted recently to try to analyse the role of DMS as a climate agent, following the original proposal of Charlson et al. (1987) in the so-called ‘CLAW’ hypothesis. Gunson et al. (2006) used a coupled ocean–atmosphere model to assess the climate sensitivity of twofold increase or decrease in DMS emissions from the oceans. They find that such an increase (which is of similar order to that found in iron fertilization experiments, as mentioned previously), if occurring globally, would produce an atmospheric temperature decrease of the order of 1–2°C, which indicates a strong sensitivity effect of DMS on global climate. In another study, Kloster et al. (in press) examine how change in climate between 1860 and the end of this century might affect DMS emissions. They find that the probable temperature increase will lead to a modelled 10% decrease in DMS, mainly due to ocean circulation changes leading to reduced productivity, which is in the opposite sense to that predicted by the CLAW hypothesis.

Study of the DMS cycle using field observations is made difficult in the Northern Hemisphere due to the natural signal being overlain by that from human-made inputs of sulphur dioxide and resulting sulphate particles, which are difficult to distinguish from the same products arising from DMS oxidation. In addition, CCN number concentrations are significantly higher in the Northern Hemisphere when compared with the Southern Hemisphere, partly due to the greater pollution sources and also due to the terrestrial dust from the larger land area. As shown originally by Twomey (1991) and recently reviewed by Lohmann & Feichter (2005), the sensitivity of climate to CCN number density is nonlinear, with the effect being much stronger at low particle numbers. For these two reasons, it is both easier and more significant to conduct field studies of the sulphur cycle in the Southern Hemisphere. For example, Vallina et al. (2006) have recently used a 3-year time-series of satellite observations of chlorophyll and CCN, rainfall amount, wind speed and model derived OH to examine the seasonality of CCN in the Southern Ocean (40–60° S). They estimate that the biogenic (DMS) contribution to CCN numbers is 35% in winter and 80% in summer, confirming the central role of marine biogenic emissions in controlling both the number density and seasonality of CCN over the remote ocean.

2. Dimethyl selenide, etc.

There are considerable similarities between the biogeochemical cycles of the Group VIb elements sulphur and selenium in the marine environment. For example, both can be methylated by micro-organisms to form volatile species which are required to balance their geochemical budgets. However, although DMS as the volatile S form has been extensively studied, there has been much less attention paid to volatile Se. One of the few studies is that of Amouroux et al. (2001), who measured volatile Se in surface waters of the North Atlantic in summer. They found that the dominant forms were DMSe (the direct analogue of DMS) and the mixed S/Se compound DMSeS, and calculate that emissions from this part of the oceans at the period of measurement were of the right order of magnitude to balance the Se budget if they applied globally. A positive relationship between seawater concentrations of DMS and DMSe was interpreted to mean that like DMS the DMSe is an algal product. On emission to the atmosphere, the volatile Se compounds will be oxidized and incorporated into particulate phases and some of this sea-to-air flux of Se will be dry and wet deposited to the land. Measurements of the Se content of mosses in Norway show decreased concentrations with distance from the sea, indicating a marine source (E. Steinnes 2003, personal communication). This may have significant implications for human health, since Rayman (2000) has shown that Se is important for human health but that people in many European countries do not reach the recommended intake levels of the element.

3. Organohalogens

Emissions of organohalogen (Br or I) gases (one to two carbon atoms) from the oceans can have significant effects on the oxidant chemistry of the atmosphere, and in some cases in particle formation. There is a growing database of shipboard measurements from which sea-to-air fluxes can be calculated (e.g. Quack & Wallace 2003; Chuck et al. 2005). However, in contrast to DMS, there is little understanding of how these compounds are formed, with both direct biological production and indirect photochemical formation from organic precursor being invoked but with little knowledge of the detailed mechanisms by which these processes operate. This uncertainty makes modelling of the effect of possible future climate change on these emissions particularly difficult. A potentially useful approach is that adopted by Gunson et al. (2006) for DMS, discussed previously, where a simple scheme for the production of the gas in the oceans is coupled to a more sophisticated atmospheric model. Then, by adjusting the oceanic emissions by an arbitrary but not unrealistic factor (2× in the case of DMS), the sensitivity of the atmospheric chemistry or climate impact to changes in emission of the gas in question can be ascertained.

The major source of organic Br from the oceans is bromoform (CHBr3) that can decompose in the atmosphere and lead to depletion of ozone in both the troposphere and the upper troposphere/lower stratosphere. Models of atmospheric chemistry have recently begun to incorporate Br chemistry and, for example, Yang et al. (2005) show that this leads to a predicted ozone loss of tropospheric ozone of approximately 5% in the Northern Hemisphere and up to 30% in the Southern Hemisphere. The situation at higher levels in the atmosphere is less certain, but Quack et al. (2004) and Salawitch (2006) argue that deep atmospheric convection of organobromine gases emitted from the tropical oceans (which are known to be prolific sources of compounds such as CHBr3) is an additional mechanism by which powerful ozone-depleting BrO radicals can reach the lower stratosphere.

For the emission from marine waters of iodine-containing organohalogen gases, it is the reactivity in the troposphere that is of importance due to their short lifetimes in air. In addition to methylated forms, such as CH3I, CH2I2 and CH2ClI, it has been reported in a recent paper that molecular iodine (I2) has been measured over seaweed beds at Mace Head, Ireland (Saiz-Lopez & Plane 2004). This is a potentially important finding due to the ability of I2 in air to form new particles and a variety of reactive iodine oxide species. However, its more general importance at other coastal sites and particularly over the open oceans has yet to be determined.

Some work has been done on the effect of increased productivity due to iron fertilization on water concentrations (and hence air–sea fluxes) of organohalogen gases as well as DMS (and CO2). Results from the EisenEx study carried out in the Southern Ocean south of Africa show the expected within-patch drawdown of CO2 and elevation of DMS, with the organohalogen CH3I increasing but with decrease in bromoform (and no significant change in methyl nitrate; Liss et al. 2005). These results illustrate the caution with which proposals to purposefully fertilize the oceans with added iron in order to sequester anthropogenic CO2 should be treated. Even if they are successful in removing carbon to depth in the oceans, we also have to take into account their effects on trace gas emissions to the atmosphere, changes to marine biodiversity and nitrous oxide production, etc. all topics of which our knowledge is inadequate.

As with Se discussed earlier, there are also human health effects to be considered for iodine. Like S and Se, the sea-to-air transfer of volatile I is important for balancing the global budget of this element. Furthermore, there is clear evidence from measurements of I in soils on transects in from the coast of deposition to the land of marine-derived iodine. Since a sufficient intake of iodine is vital for human health and it has been estimated that 38% of the world population suffer from some form of iodine deficiency disorder (Johnson et al. 2003), this is clearly a topic worthy of further study (along with the equivalent Se cycle).

4. Alkyl nitrates, oxygenated volatile organic compounds, isoprene

Alkyl nitrates, specifically methyl and ethyl nitrate, are significant components of the ‘odd nitrogen’ (NOy) component of the atmosphere, where they play important roles in regulating ozone levels. The alkyl nitrates were until recently thought to be essentially of anthropogenic origin. However, Chuck et al. (2002) reported measurements of these compounds in surface seawater and air from the North and South Atlantic that indicate a (natural) flux out of the oceans, which was particularly strong in equatorial waters. Although production processes for trace gases in seawater are often poorly known or totally unknown, in the case of these alkyl nitrates laboratory experiments indicate that they can be formed photochemically by the reaction of ROO (R=alkyl group) and NO (Dahl et al. 2003).

Oxygenated volatile organic compounds are a group of oxygen-containing organic gases that play important roles in the oxidant chemistry of the troposphere by sequestering reactive nitrogen oxides and acting as a source of free radicals. Much of what we know about their atmosphere–ocean exchange behaviour is obtained by inference from their atmospheric distributions. Singh et al. (2001) pointed out that in order for the modelled vertical distributions of several of these gases to match their measured atmospheric profiles, an additional source was required and speculated that this could be emission from the oceans. An exception seems to be methanol where atmospheric measurements indicate a flux into the oceans (Carpenter et al. 2004). The only direct flux estimates involving measurements in surface seawater and the overlying air are those of Williams et al. (2004), who found a flux out of the oceans of acetonitrile and acetone, whereas the flux of methanol was from air to sea.

Isoprene (C5H8) is a gas reactive in the atmosphere, particularly important for the formation of organic particles. It is known to be emitted from the oceans (Broadgate et al. 1997), but generally not thought to be of sufficient magnitude to lead to particle formation. However, in a recent paper, Meskhidze & Nenes (2006) have used satellite imagery to show that over a biological productive region of the Southern Ocean, CCN number density and cloudiness are enhanced compared with the less productive surrounding area. Controversially, they attribute the additional CCN to particles formed from isoprene emitted from the biologically productive water, where it might be expected that DMS would be more obvious precursor. This finding is of importance not only because it shows that marine trace gas emissions can have a direct effect on CCN and cloudiness and hence climate, but also reopens the question of the role of oceanic isoprene emissions on the marine atmosphere.

5. Conclusions

For trace gases considered here, it is clear that marine emission plays significant roles in the chemistry of the atmosphere and processes affecting the climate, as well as in cycling elements to land which are of importance for human health. In a number of cases, we lack even basic knowledge of how the gases are formed (and destroyed) in the surface oceans and this is a real handicap in trying to predict how the fluxes may change under an altered climate. What is clear is that multidisciplinary approaches, for example, combining remotely sensed data with that collected at sea level, and comparison between field measurements and model predictions are powerful ways forward.

Acknowledgments

I thank several members of my research group for their suggestions as to which papers should be included here.

Footnotes

One contribution of 18 to a Discussion Meeting Issue ‘Trace gas biogeochemistry and global change’.